In cases where high-speed, large-capacity optical transmission paths are constructed by using optical transmission technology, a nonlinear transmission characteristic called chromatic dispersion exerts a great influence on the performance.
Chromatic dispersion is a phenomenon in which as light is propagated through an optical fiber, its waveforms spread along the time axis. To permit optical signals to be transmitted over a long distance without entailing distortion, the chromatic dispersion caused needs to be sufficiently small.
Meanwhile, in recent years, optical transmission systems have been under development whereby optical signals with different bit rates (e.g., 10 Gb/s and 40 Gb/s) can be transmitted. In such applications, the influence of chromatic dispersion on a 40-Gb/s optical signal is approximately 16 times as much as that of chromatic dispersion on a 10-Gb/s optical signal. Thus, in systems handling 10- and 40-Gb/s optical signals, it is necessary that the chromatic dispersion be efficiently compensated with respect to both of the bit rates.
To compensate chromatic dispersion, a method is generally adopted in which, for example, dispersion compensation modules (DCMs) tuned to the respective bit rates are inserted into an optical fiber to perform dispersion compensation.
As conventional dispersion compensation techniques, Japanese Laid-open Patent Publication No. 2005-295126 (paragraph nos. [0021] to [0038], FIG. 11) identified below proposes a technique in which a compensation node is arranged in each in-line relaying apparatus, optical signals are subjected to wavelength separation according to bit rates in the compensation node, and an dispersion compensation amount is set with respect to each bit rate.
In optical transmission systems such as the aforementioned ones, multi-wavelength transmission is carried out by using WDM (Wavelength Division Multiplex) technology. Also, control referred to as OADM (Optical Add and Drop Multiplexing) is executed in which, in compliance with a communication request of each node, optical signals of desired wavelengths are inserted (added) into and extracted (dropped) from desired paths.
However, where the dispersion is compensated in conventional optical transmission systems in which optical signals with different bit rates are subjected to OADM, DCMs tuned to the respective bit rates have to be inserted into each add/drop optical fiber via which optical signals are added or dropped, which leads to enlargement of the scale of the system and increase of costs.
Practical application of optical transmission systems for 10- and 40-Gb/s optical signals has just begun at this stage, and a technique for efficiently compensating the chromatic dispersion of the individual bit rates has not been established yet. Accordingly, there has been an urgent demand for techniques applicable to systems for performing OADM control on optical signals with different bit rates and capable of efficiently compensating the chromatic dispersions of the optical signals with the respective different bit rates.